Cobalt-60 External Beam Radiation Therapy

Introduction to Teletherapy&

Cobalt Machine

Teletherapy

Classification

● 1. Grenz Therapy● 2. Contact Therapy● 3. Superficial Therapy● 4. Orthovoltage Therapy● 5. SuperVoltage Therapy● 6. MegaVoltage Therapy

Clinical Radiation Generators

● Early external beam radiation therapy was carried out with voltages up to 300kV

● Since then radiation therapy units are in the MeV ranges (From Cobalt-60 to Linacs)

● However there are still some low energy beams used for the treatment of skin lesions.

Grenz Therapy● Has a very soft beam (<20kV).● Absorbed within the first 2 mm

of skin - does not penetrate beneath the dermis.

● Used clinically for skin lesions like Atopic Dermatitis, Psoriasis, Lichen Planus, Acne.

● Typically 200 cGy per session at weekly intervals for a total of 800cGy to 1000cGy

Contact Therapy

● 40-50 kV small focal spot, short SSD (~5 cm)● Absorbed with 2 cm of tissue.● SSD is 2 cm or less.● Also used for endocavitary irradiation.

(Selected Rectal, Oral Kaposi Sarcoma etc)

Superficial Units (SXRT)

● 50-150 kV. SSD: 15/20 cm. ● Treatment option for skin

tumors of 5.0 mm depth including BCC, SCC or Kaposi’s sarcoma.

● The beam energy penetrates only the top surface layer of the skin.

Orthovoltage (Deep) Units

● 150 - 500 kV.● Treatment fields used to be defined using

detachable cones. ● The SSD was typically 50 cm.● Not in 'clinical' use.

Supervoltage Therapy

● This is therapy units with x-ray ranges from 500-1,000kV

● Due to the demand of treating deeper tumors these units were created.

● Since conventional power transformers were not suitable for high energy units (>300kV) so for new machines were created to facilitate this.

● Resonant Transformers were created which step up the voltage in an efficient manner

Megavoltage Therapy

● X-rays beam of energy 1 MV or greater.● Van de Graff generators, Linacs,Betatron,

Microtron, Teletherapy, Radioisotope Units● Though not x-rays, radionuclides with γ rays of

1MeV or greater are also included.

Van de Graaff Generator

● Electrostatic accelerator used to accelerate charged particles.

● Typically a 20-40 kV is applied to a moving belt of insulatator.

● The electrons are carried to the top of the belt, and are removed by a collector.

● A large collection of negative is accumulated at the top of dome.

● When applied across the x-ray tube. This allows production of x-rays when electrons strike the target.

● Van de Graff is capable of reaching energies of 10 MV, but limited by size, and insulation

● Not used clinically due to emergence of Cobalt-60 units, and Linacs .

A 7 eMV Van de Graaff accelerator at JRC-IRMM

Betatron● Electron in a changing magnetic field

accelerate in a circular orbit.● An electron pulse is introduced into the donut,

between poles of a magnet of AC current.● Not used due to small field size and dose rate

capabilities.

Betatron 6MeV (1942)

Microtron

● Electrons are accelerated by oscillating fields of 1 or more microwave cavities

● Then a magnetic field forces electrons to orbit in a circular path and return to the cavity.

● Lower electric consumption● Simpler cooling system● Low-cost

Cyclotron● Charged particle acclerator, mainly used for

nuclear physics research.● The High Voltage, High frequency Occilator is

used to acclerate the particle ● Used as a source of high energry proton beam

therapy.● Also can be used as neutron beam

(after suitable target being hit)

Machines Using Radionuclides

● Radium-226● Cesium-137● Cobalt-60

● Cobalt-60 is suitable for EBRT:1. Higher possible specific activity (curie/gram)

2. Greater Radiation Output per curie

3. Higher average Photon energy

● 59-Co is irradiated with Neutrons in a reactor.● Takes 5 to 10 years to produce.● First done in London, Ontario, Canada.

Cobalt-60

Source

● Source is usually a solid cylinder (1 x 2.5cm), and encapsulated in a stain-less steel capsule.

Head

● Shields the source, Exposes the source as required, and Collimates the beam to the correct size.

Source Shielding

● Consists of a steel shell with lead for shielding purposes and a mechanism for bringing the source in front of the collimator opening to produce the clinical gamma ray beam.

● The source moves into 'Beam On' region from the 'Beam Off' region by either:(i) a source on a sliding drawer and (ii) a source on a rotating cylinder.

Collimator and penumbra

● Collimators provide square and rectangular radiation fields typically ranging from 5 × 5 to 35 × 35 cm at 80 cm from the source.

● The geometric penumbra, which results from a finite source diameter, may be minimized by using small diameter sources and by using penumbra trimmers as close as possible to the patient’s skin.

Gantry

● The 'thing' that moves around an 'isocentric' point is the Gantry.

● Made so because once set, the SSD doesn't change with the circular movement of Gantry.

ABSTRACT:The simplicity of cobalt units gives them the advantage of reduced maintenance, running costs and downtime when compared with linear accelerators.However, treatments carried out on such units are typically limited to simple techniques. This study has explored the use of cobalt beams for conformal and intensity-modulated radiotherapy (IMRT).

If cobalt units were to have such features incorporated into them, they could offer considerable benefits to the radiotherapy community.

The British Journal of Radiology, 81 (2008), 304–310

Cobalt Vs Linear Acclerator